PROJECT SUMMARY Non-steroidal anti-inflammatory drugs (NSAIDs) are a first line pharmacologic pain therapy for chronic musculoskeletal pain, and rheumatoid arthritis (RA) and moderate to severe osteoarthritis (OA) specifically. However, insufficient pain relief by NSAID monotherapy has encouraged the use of combination therapy. Combinations of NSAIDs plus weak opioids are widely used although objective evidence for efficacy is limited and they have many adverse events. With the opioid crisis in the United States being declared a national public health emergency, the need for new, effective, and non-addictive pain medications has never been greater. Next generation management of chronic musculoskeletal pain must relieve both acute pain components, caused by structural joint damage, and chronic pain components, caused by disinhibition of central pain regulatory mechanisms. This is substantiated by the many RA patients (~12%) that continue to suffer from chronic pain even though disease-modifying antirheumatic drugs (DMARDs) can effectively reduce inflammatory pain symptoms. Furthermore, disinhibition of central pain regulatory mechanisms not only plays a key role in chronic musculoskeletal pain, but also leads to increased risk of patients developing chronic widespread pain state comorbidities such as fibromyalgia (FMS), which affect up to 20% of RA patients and 11% of OA patients. A growing body of evidence suggests that ?2/?3 subtype-selective positive allosteric modulators (PAM) of the ?- aminobutyric acid A receptor (GABAAR) may effectively restore central pain regulatory mechanisms thus providing effective relief of chronic pain with reduced prevalence and severity of side-effects. Recently, NeuroCycle Therapeutics (NCT) collaborator, Prof. Hanns Ulrich Zeilhofer at University of Zurich, Switzerland, demonstrated that TPA-023B, an ?2/?3 subtype-selective PAM, significantly reduced the aversive component of non-inflammatory pain (chronic constriction injury-induced neuropathic pain) using conditioned place preference (CPP). Furthermore, TPA-023B robustly reduced Mouse Grimace Scale Scores in a formalin test in mice, a valid model for acute and tonic pain. TPA-023B is a small molecule with exquisite specificity and potency at ?2/?3 subtypes; excellent oral bioavailability and tolerability in humans; and has demonstrated target engagement in man. Based on these promising preliminary studies and considerable supporting literature data, we propose a SBIR Phase I project to test the hypothesis that combination dosing of TPA-023B with an NSAID will work synergistically to suppress the acute and chronic pain components of chronic musculoskeletal pain. Our specific aims are: 1) To investigate the efficacy of TPA-023B + NSAID combination in chronic pain relief in complete Freund's adjuvant (CFA)-induced arthritis in rats; 2) To investigate the efficacy of combination therapy in chronic pain relief in monosodium iodoacetate (MIA)-induced arthritis in rats; 3) To investigate the efficacy of combination therapy in widespread pain relief in acid saline-induced pain in rats. The results will provide strong support for a Phase II project, where additional pharmacology studies and IND-enabling formulation development will be performed in preparation for initial clinical trials in man.